1. Field of the Invention
The invention relates to a transceiver, and in particular to a transceiver which has a function of fast switch between receive and transmit implemented by using different frequencies of intermediate frequency (IF) signals.
2. Description of the Prior Art
Today, communication technology, especially in radio communication systems, such as cellular phones and satellite communication, has been widely and swiftly developed and used. Generally, in low-tier personal digital communication systems, variety of modulation schemes can be grouped into IQ modulation systems and non-IQ modulation systems. The way for multiplex includes a time-division duplex (TDD) and a frequency-division duplex (FDD). In generally, to implement GFSK, modulation schemes are classified into Quadrature Modulation and Direct VCO Modulation. In the invention, GFSK modulation schemes applied in the non-IQ modulation system is adopted.
Typically, an IF block used for processing modulation and demodulation signals is included in chip sets in a personal communication system. In the transmission path, the IF block can convert a digital input signal to an analog modulated output signal that serves as an intermediate frequency input signal. In the Rx path, the IF block can also down convert and demodulate a receive signal into a baseband signal. Next, a conventional transceiver will be provided for description.
Referring to FIG. 1, a circuit block diagram of a conventional transceiver 14a is shown. The conventional transceiver 14a includes a receiver 102, a transmitter 104a, a band pass filter 106 and a receive/transmit switch 108. The receiver 102 includes a Mixer and IF AMP 110 and a discriminator 112. The transmitter 104a includes a phase-lock-loop (PLL) 114, a transmit signal switch 116 and a power amplifier 118.
As to a receiving operation thereof, a modulated receive signal 20a received from an antenna 12 is filtered by the band pass filter 106 and then delivered to the receive/transmit switch 108. According to a receive/transmit control signal 30f, the receive/transmit switch 108 feeds the filtered and modulated receive signal 122 into the Mixer and IF AMP 110. At this time, according to an enable/disable signal 30a, the Mixer and IF AMP 110 can down convert the receive signal 122 to a Rx IF signal 126 in coordination with a local oscillation signal 124. The discriminator 112 receives the Rx IF signal 126, and then generates a demodulated receive signal 22a. 
In TDD mode, the Mixer and IF AMP 110 of the receiver 102 and the discriminator 112 can receive the enable/disable signal 30a for enabling/disabling.
Similarly, the transmitter 104a receives enable/disable signals 30b, 30c and 30d. The phase-lock-loop 114 can receive a PLL enable/disable signal 30b to enable all active devices. The phase-lock-loop 114 can receive a VCO enable/disable signal 30c to enable/disable a voltage-controlled oscillator (VCO). The power amplifier 118 can be enabled/disabled according to an enable/disable signal 30d. 
The phase-lock-loop 114 included in the transmitter 104a is used to generate a radio frequency signal 130. According to the receive/transmit control signal 30f, a required radio frequency signal 130 is selected by the transmit signal switch 116. When the transceiver 14a is at a transmit mode, a radio frequency signal 132 output from the transmit signal switch 116 is fed into the power amplifier 118, and then amplified. Afterwards, the amplified radio frequency signal 134 is fed into the receive/transmit switch 108. According to the receive/transmit control signal 30f, the radio frequency signal 134 is filtered by the filter 106, and then transmitted from the antenna 12.
On the contrary, when the transceiver 14a is at a receive mode, a radio frequency signal 130 generated by the phase-lock-loop 114 is selected to serve as the local oscillation signal 124 of the receiver 102 by the transmit signal switch 116, and then fed into the Mixer and IF AMP 110 of the receiver 102.
As described above, the transmitter 104a and receiver 102 of the transceiver use the same intermediate frequency signal. Therefore, under a transmit mode, it is necessary to perform an open loop operation for preventing distortions caused to the modulated signal by the phase-lock-loop. Furthermore, the time for the open loop modulation system to reach a stable state cannot keep longer, leading to influence to data transmission amount. As a result, a system reaction delay is usually created to lower data transmission amount.
In view of the above, an object of the invention is to provide a transceiver with a receive/transmit fast switch function which can efficiently prevent interference caused during a signal receive/transmit operation by using different receive/transmit intermediate frequency signals. Additionally, the transceiver of the invention can also eliminate data distortion, lengthen transmission slot and decrease the stand-by time for receive/transmit switches.
The operation of the transceiver with a receive/transmit fast switch function according to the invention will be briefly described hereinafter.
Under a transmit mode, a carrier signal is generated and modulated by a frequency generator, and then fed into a transmitter. Inversely, under a receive mode, a frequency-fixed carrier signal is generated by the frequency generator. Moreover, a first intermediate frequency signal and a second intermediate frequency signal are generated by an intermediate frequency circuit, and then fed into the transmitter and the receiver to serve as local oscillation signals. When transmitting a signal, the modulated carrier signal and the first intermediate frequency signal are received by the transmitter. Then, the modulated carrier signal is up converted by a mixer, and then outputted. Oppositely, when receiving a signal, the receive signal is down converted by a mixer in coordination with the second intermediate frequency signal and the carrier signal, and then outputted. Finally, the receive signal is demodulated into an FSK signal by a discriminator. The transceiver frequency synthesizer is no longer re-set or changed into any frequency between Tx/Rx mode except that a new RF channel is allocated.